1. Field of the Invention
The present invention relates to a silicon-containing flame retardant. More particularly, the present invention is concerned with a novel silicon-containing flame retardant for an organic polymer, comprising a monomer, a polymer or a mixture thereof, which is represented by the following formula (1): 
wherein each of R1 and R2 independently represents a monovalent C1-C20 hydrocarbon group; each of R3 and R4 independently represents a monovalent or divalent C1-C20 hydrocarbon group, with the proviso that, when each of R3 and R4 independently represents a divalent C1-C20 hydrocarbon group, R3 and R4 are bonded to each other to form a ring; and n is 1 or more in terms of the number average n-value.
The present invention is also concerned with a method for imparting a flame retardancy to an organic polymer, which comprises adding to an organic polymer the above-mentioned silicon-containing flame retardant.
When the silicon-containing flame retardant of the present invention is added to an organic polymer, the resultant composition exhibits not only excellent flame retardancy but also excellent impact strength and heat resistance.
2. Prior Art
Polymers, such as synthetic resins and elastomers, exhibit excellent moldability and high impact resistance, as compared to glass materials and metals. Therefore, polymers have been used in various fields, such as automobile parts, parts for household electric appliances and parts for office automation machines. However, the use of the polymers is limited due to the flammability thereof. Therefore, a number of proposals have been made with respect to the method for imparting flame retardancy to a polymer.
As a method for imparting a flame retardancy to a polymer, there is known a method in which a flame retardant, such as a halogen-containing flame retardant, a phosphorus-containing flame retardant or an inorganic flame retardant, is added to a polymer.
However, the conventional flame retardants have the following problems. The halogen-containing flame retardant is unfavorable from the viewpoint of the protection of environment. Specifically, the halogen-containing flame retardant generates an acidic gas during the burning thereof, and the generated acidic gas becomes causative of disadvantageous phenomena, such as corrosion of a combustion furnace and occurrence of acid rain. Further, the halogen-containing flame retardant is difficult to recycle. On the other hand, the phosphorus-containing flame retardant is hydrolyzable. Therefore, when a shaped article containing a phosphorus-containing flame retardant is exposed to the weather, the phosphorus-containing flame retardant bleeds out from the shaped article and hydrolyzed, thereby causing eutrophication. Further, the phosphorus-containing flame retardant is also difficult to recycle.
With respect to the polymers containing the phosphorus-containing flame retardant and/or the inorganic flame retardant, the heat stability, impact strength, melt-fluidity and heat resistance thereof are not always satisfactory, so that the industrial use of such polymers is limited.
Further, in recent years, there has been a strong demand for improving safety of materials against fire. In order to meet such a strong demand, improved techniques for imparting flame retardancy to polymers have been developed. However, such flame retardancy-imparting techniques are accompanied by problems, such as harmful effects on the environment and a lowering of the mechanical properties of the polymers.
Under these circumstances, flame retardancy-imparting techniques using a silicon-containing flame retardant, which is not accompanied by the above-mentioned problems, have been developed.
As an example of silicon-containing flame retardants, a silicone or silicone resin, such as dimethyl-silicone, is known. A flame retardant resin composition containing a silicone or silicone resin is free from the above-mentioned problems (regarding the recycling and the environment) accompanying the use of the halogen- or phosphorus-containing flame retardant. However, the flame retardancy of the flame retardant resin compositions containing a silicone or a silicone resin is poor, and, therefore, cannot be put into practical use.
As another example of silicon-containing flame retardants, an organosilicate is known. For example, a flame retardant resin composition comprising an aromatic polycarbonate and a silicate resin is disclosed in EP 415070 A2 (corresponding to Unexamined Japanese Patent Application Laid-Open Specification No. 3-143951). However, the above-mentioned organosilicate resin has a branched structure and a crosslinked structure and, hence, exhibits a poor dispersibility in a resin mixed with the organosilicate resin. Further, the performance of this organosilicate resin as a flame retardant is not satisfactory.
Unexamined Japanese Patent Application Laid-Open Specification No. 9-111109 discloses a flame retardant resin composition composed of a polycarbonate resin, polymethylethoxysiloxane, titanium oxide, polytetrafluoroethylene, a halogen-containing flame retardant and a thermoplastic elastomer. The organoalkoxypolysiloxane (i.e., polymethylethoxysiloxane) used as a flame retardant in this resin composition is an organosilicate which contains Sixe2x80x94Oxe2x80x94R linkages and Sixe2x80x94R linkages (each R is a hydrocarbon group). Due to the presence of the Sixe2x80x94R linkages, the organoalkoxypolysiloxane cannot impart a satisfactory flame retardancy to a resin.
Further, EP 771835A2 (corresponding to Unexamined Japanese Patent Application Laid-Open Specification Nos. 9-165452 and 9-165451) and Unexamined Japanese Patent Application Laid-Open Specification Nos. 7-48454 and 9-208744 disclose resin compositions containing a linear organosilicate. However, each of the resin compositions disclosed in these patent documents is a coating composition, in which the linear organosilicate is used merely as a curing agent.
As can be seen from the above, there has conventionally been no report that the linear organosilicate can be used for imparting a polymer with excellent properties, such as high flame retardancy, high impact strength and high heat resistance.
In this situation, the present inventors have made extensive and intensive studies with a view toward developing a flame retardant which is free from the above-mentioned problems and can be used for imparting an excellent flame retardancy to a polymer.
As a result, it has unexpectedly been found that, when a specific, silicon-containing flame retardant described below is used, not only can a flame retardancy of an organic polymer be greatly improved, but also an organic polymer can be imparted with high impact strength and high heat resistance. The above-mentioned specific, silicon-containing flame retardant comprises a monomer, a polymer or a mixture thereof, which is represented by the following formula (1): 
wherein:
each of R1 and R2 independently represents a monovalent C1-C20 hydrocarbon group;
each of R3 and R4 independently represents a C1-C20 alkyl group or a monovalent C6-C20 aromatic group or independently represents a C1-C20 alkylene group or a divalent C6-C20 aromatic group, with the proviso that, when each of R3 and R4 independently represents a C1-C20 alkylene group or a divalent C6-C20 aromatic group, R3 and R4 are bonded to each other to form a ring; that all of R1, R2, R3 and R4 are not simultaneously alkyl groups or monovalent aromatic groups; that, when each of R1 and R2 is an alkyl group, both of R3 and R4 are not simultaneously alkylene groups; and that, when each of R1 and R2 is a monovalent aromatic group, both of R3 and R4 are not simultaneously divalent aromatic groups;
n is 1 or more in terms of the number average n-value.
The present invention has been completed, based on this novel finding.
Therefore, it is a primary object of the present invention to provide a silicon-containing flame retardant which can be used for imparting an organic polymer with not only excellent flame retardancy but also high impact strength and high heat resistance.
Another object of the present invention is to provide a method for imparting an organic polymer with flame retardancy, which comprises adding the above-mentioned silicon-containing flame retardant to an organic polymer.